In the killer whale world, a noisy seal makes a fine meal, says new research into the hunting practices of transient orcas off the Pacific coast.
A study by the Vancouver Aquarium a few years ago revealed that transient orcas do not, as once believed, hunt using echolocation to target prey, measuring the bounce of their own calls back to them in order to hone in on dinner. That left biologists wondering just how they do hunt.
"Because transient don't echolocate, we did not know what senses they use to find prey," said Volker Deecke, a researcher at the Centre for Wildlife Conservation at the University of Cumbria in Britain and a former research associate at the Vancouver Aquarium.
The waters off British Columbia and Alaska are home to two types of killer whales – the resident whales that feed solely on fish, and the transient whales that travel the waters more freely and feed on other mammals like porpoise and harbour seals.
Using acoustic recording tags stuck to 13 transient killer whales with suction cups, the team led out of Cumbria recorded the mammals in the ocean off the coast of Alaska for up to 16 hours at a time.
They wanted to know if the whales were hunting with their sight, picking prey silhouetted against the surface, or listening for their soon-to-be lunch to make some noise.
"Our study found that transient killer whales frequently caught prey in near-complete darkness," Mr. Deecke said in an e-mail to The Canadian Press.
It told the team that the whales were not relying on vision, he said.
The group suspected the whales were using their keen hearing to target their next meal and a juvenile orca provided some compelling evidence as he hunted all night in a glacial fiord off southeast Alaska.
Soon after tagging, the tag's hydrophones picked up the distant roars of a male harbour seal trying to attracted a mate.
"Over the next 30 minutes the roars got louder until at some point the tag recorded three loud roars where the whale must have been within a few hundred metres of the seal at the most," Mr. Deecke said.
"Exactly 27 seconds later, the tag recorded all the signs of a predation event – the sounds of whales ramming the prey, bones snapping and flesh ripping. No more roars after that."
While not direct evidence that the transient whales eavesdrop to find food, it's an important clue in the puzzle, suggested Lance Barrett-Lennard, the marine mammal scientist at the Vancouver Aquarium who conducted the earlier research on echolocation.
"We, as humans, are putting a lot of noise in the water. Sound carries so well in water, it carries great distances," he said.
"A critter like a transient killer whale, that makes a living by listening for very quiet sounds, is at a real disadvantage when we humans introduce a lot of noise."
Noise from boats and other human activity also affects resident orcas, but they're generally more chatty and ramp up their own vocalizations to compensate for noise, he said.
"But transients don't do that," Mr. Barrett-Lennard said. "They're just passively listening for the sounds of prey, so we think the boat noise is a much more serious problem for transients."